TDA7402 datasheet(47/71 Pages) STMICROELECTRONICS

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TDA7402

Functional description of the multipath detector

47/71

Functional description of the multipath detector

Using the multipath detector the audible effects of a multipath condition can be minimized. A

multipath condition is detected by rectifying the spectrum around 19kHz in the fieldstrength

signal. An external capacitor is used to define the attack and decay times for the stereo

blend (see block diagram,

Figure 34). Due to the very small charge currents this capacitor

should be a low leakage current type (e.g ceramic). Using this configuration an adaptation to

the user's requirement is possible without effecting the "normal" fieldstrength input (level) for

the stereo decoder. This application is given in

Figure 34. Another (internal) time constant is

used to control the Highcut through the multipath detector

Selecting the "internal influence" in the configuration byte the stereo-blend and/or the

highcut is automatically invoked during a multipath condition according to the voltage

appearing at the MP_OUT pin.

Figure 35.

Block diagram of the multipath detector

8.1

Quality detector

The A619 offers a quality detector output which gives a voltage representing the FM

reception conditions. To calculate this voltage the MPX-noise and the multipath-detector

output are summed according to the following formula :

VQual = 1.6 (VNoise-0.8 V)+ a (REF5V-VMpout).

The noise-signal is the PEAK signal without additional influences (see the noiseblanker

description). The factor 'a' can by programmed to 0.7 .... 1.15. The output is a low

impedance output able to drive external circuitry as well as simply fed to an AD converter for

RDS applications.

INT.

INFLUENCE

CHARGE

0.25

μA/0.5μA

FAST

CHARGE

300

μA

LEVEL

D00AU1166

LOWPASS

2.2KHz

BANDPASS

19KHz

RECTIFIER

SHIFT

0 ... 1.5V

LOWPASS

100Hz

LOWPASS

53KHz

INT.

INFLUENCE

MP HOLD

GAIN 2 bits

MP_IN

to HCC

to SB

470nF

MP_OUT

VDD

High-Cut-Infl.

Attach<500

μs

Decay=2/10ms

Stereoblend-Infl.

Attach=1ms

Decay=4/8s

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